The present invention relates to a drainage system and more particularly to a drainage material and to a mesh structure drainage core therefor. The drainage core comprises two sets of generally parallel strands in a bi-planar configuration, by which is meant that the median plane of one set is displaced from the median plane of the other set. The drainage material has a pervious or impervious member against one face, and in use in the drainage system will normally have a pervious member against the other face. The pervious member normally comprises filter material in order to keep soil particles etc out of the drain. Such drainage materials and drainage systems are known, and U.S. Pat. No. 3,371,785, U.S. Pat. No. 3,563,038 and U.S. Pat. No. 3,654,765 describe such systems; GB-A-2 002 686 describes a specific drainage material in the form of a bi-planar plastics material mesh core laminated with a non-woven fabric filter material. Such cores are conventionally made by integral extrusion in a plastics material, and integrally-extruded mesh structures are inexpensive and corrosion resistant and are convenient to store, transport and use. The cores are symmetrical about their median plane, i.e. the strand heights are substantially equal.
There are three main types of drainage system in which the drainage materials are used:
(i) As slit drains in soil in each case positioned roughly vertically in the soil. "Vertical drains" lead down to a duct at the bottom for the collection of water and "horizontal drains" are for carrying the water away generally horizontally. Normally, the drainage core will have a pervious member against both faces.
(ii) On the sides and bottoms of waste management pits, for draining water to one or more sumps in the bottom. One such waste management pit has an impervious lining on which is placed the drainage core with a pervious material on top of the mesh structure. However, there can be a lower layer as a safety measure, in which there is a lower, impervious lining with a mesh structure core on top, overlain by the next impervious lining. On the sides, which usually slope, it is most efficient for the water to flow straight down the fall line, but on the bottom multi-directional flow may be required to enable the water to reach the sump(s).
(iii) Against foundations of houses or bridges. In this case, the drainage core can be immediately against the foundation, with a pervious member on the outside, to drain ground water away from the foundations.
The bi-planar mesh structure core provides channels between the strands for the flow of water within the mesh structure, and the outer faces of the strands also provide a support against which the pervious or impervious member rests. Such mesh structure cores are of diamond form, i.e. the sets of strands run each at an angle to the intended flow direction, and the openness of the structure was considered important in enabling the flow to pass from one plane to the other. However bi-planar cores do not have an ideal configuration and have poor crush resistance. Considerable efforts have been made in order to increase the drainage capacity of such cores. This is done by increasing the heights of the strands, and suitably thickening the strands in order to prevent them bending over or collapsing. Although greater flow capacities can be achieved in this way, the flow capacity per unit mass of material used in the mesh structure does not increase very impressively.